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Gastrointestinal (GI) discomfort is a hallmark of most gut disorders and represents an important component of chronic visceral pain1. For the growing population afflicted by irritable bowel syndrome, GI hypersensitivity and pain persist long after tissue injury has resolved2. Irritable bowel syndrome also exhibits a strong sex bias, afflicting women three times more than men1. Here, we focus on enterochromaffin (EC) cells, which are rare excitable, serotonergic neuroendocrine cells in the gut epithelium3-5. EC cells detect and transduce noxious stimuli to nearby mucosal nerve endings3,6 but involvement of this signalling pathway in visceral pain and attendant sex differences has not been assessed. By enhancing or suppressing EC cell function in vivo, we show that these cells are sufficient to elicit hypersensitivity to gut distension and necessary for the sensitizing actions of isovalerate, a bacterial short-chain fatty acid associated with GI inflammation7,8. Remarkably, prolonged EC cell activation produced persistent visceral hypersensitivity, even in the absence of an instigating inflammatory episode. Furthermore, perturbing EC cell activity promoted anxiety-like behaviours which normalized after blockade of serotonergic signalling. Sex differences were noted across a range of paradigms, indicating that the EC cell-mucosal afferent circuit is tonically engaged in females. Our findings validate a critical role for EC cell-mucosal afferent signalling in acute and persistent GI pain, in addition to highlighting genetic models for studying visceral hypersensitivity and the sex bias of gut pain.

Functional bowel disorder patients can suffer from chronic abdominal pain, likely due to visceral hypersensitivity to mechanical stimuli. As there is only a limited understanding of the basis of chronic visceral hypersensitivity (CVH), drug-based management strategies are ill defined, vary considerably, and include NSAIDs, opioids, and even anticonvulsants. We previously reported that the 1.1 subtype of the voltage-gated sodium (NaV; NaV1.1) channel family regulates the excitability of sensory nerve fibers that transmit a mechanical pain message to the spinal cord. Herein, we investigated whether this channel subtype also underlies the abdominal pain that occurs with CVH. We demonstrate that NaV1.1 is functionally upregulated under CVH conditions and that inhibiting channel function reduces mechanical pain in 3 mechanistically distinct mouse models of chronic pain. In particular, we use a small molecule to show that selective NaV1.1 inhibition (a) decreases sodium currents in colon-innervating dorsal root ganglion neurons, (b) reduces colonic nociceptor mechanical responses, and (c) normalizes the enhanced visceromotor response to distension observed in 2 mouse models of irritable bowel syndrome. These results provide support for a relationship between NaV1.1 and chronic abdominal pain associated with functional bowel disorders.

Dietary, microbial, and inflammatory factors modulate the gut-brain axis and influence physiological processes ranging from metabolism to cognition. The gut epithelium is a principal site for detecting such agents, but precisely how it communicates with neural elements is poorly understood. Serotonergic enterochromaffin (EC) cells are proposed to fulfill this role by acting as chemosensors, but understanding how these rare and unique cell types transduce chemosensory information to the nervous system has been hampered by their paucity and inaccessibility to single-cell measurements. Here, we circumvent this limitation by exploiting cultured intestinal organoids together with single-cell measurements to elucidate intrinsic biophysical, pharmacological, and genetic properties of EC cells. We show that EC cells express specific chemosensory receptors, are electrically excitable, and modulate serotonin-sensitive primary afferent nerve fibers via synaptic connections, enabling them to detect and transduce environmental, metabolic, and homeostatic information from the gut directly to the nervous system.

Voltage-gated sodium (Na.sub.v) channels initiate action potentials in most neurons, including primary afferent nerve fibres of the pain pathway. Local anaesthetics block pain through non-specific actions at all Na.sub.v channels, but the discovery of selective modulators would facilitate the analysis of individual subtypes of these channels and their contributions to chemical, mechanical, or thermal pain. Here we identify and characterize spider (Heteroscodra maculata) toxins that selectively activate the Na.sub.v1.1 subtype, the role of which in nociception and pain has not been elucidated. We use these probes to show that Na.sub.v1.1-expressing fibres are modality-specific nociceptors: their activation elicits robust pain behaviours without neurogenic inflammation and produces profound hypersensitivity to mechanical, but not thermal, stimuli. In the gut, high-threshold mechanosensitive fibres also express Na.sub.v1.1 and show enhanced toxin sensitivity in a mouse model of irritable bowel syndrome. Together, these findings establish an unexpected role for Na.sub.v1.1 channels in regulating the excitability of sensory nerve fibres that mediate mechanical pain. Two spider toxins are shown to target the Na.sub.v1.1 subtype of sodium channel specifically, shedding light on the role of these channels in mechanical pain signalling. Na.sub.v1.1 channels mediate mechanical pain Mutations affecting several Na.sub.v1 subtype voltage-gated sodium channels have been shown to be associated with insensitivity to pain or persistent pain syndromes. Na.sub.v1.1 is expressed by somatosensory neurons, but no direct link has been established between this subtype and nociception. Further studies have been hampered by a paucity of pharmacological agents that discriminate between the closely related members of the Na.sub.v1 family. Now David Julius and colleagues have identified two spider toxins specifically targeting Na.sub.v1.1, and use them to show that this channel is key to the specific transduction of mechanical but not thermal pain by myelinated A[delta] sensory fibres. Previous genetic studies of Na.sub.v1.1 indicate that such selective agents may open therapeutic avenues in disorders associated with the central nervous system, such as epilepsy, autism and Alzheimer disease. The involvement of Na.sub.v1.1 channels in mediating mechanical pain reported here was unexpected., Author(s): Jeremiah D. Osteen [sup.1] , Volker Herzig [sup.2] , John Gilchrist [sup.3] , Joshua J. Emrick [sup.1] , Chuchu Zhang [sup.1] , Xidao Wang [sup.4] , Joel Castro [sup.5] [...]

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a common chronic pelvic disorder with sensory symptoms of urinary urgency, frequency, and pain, indicating a key role for hypersensitivity of bladder-innervating sensory neurons. The inflammatory mast cell mediator histamine has long been implicated in IC/BPS, yet the direct interactions between histamine and bladder afferents remain unclear. In the present study, we show, using a mouse ex vivo bladder afferent preparation, that intravesical histamine enhanced the mechanosensitivity of subpopulations of afferents to bladder distension. Histamine also recruited “silent afferents” that were previously unresponsive to bladder distension. Furthermore, in vivo intravesical histamine enhanced activation of dorsal horn neurons within the lumbosacral spinal cord, indicating increased afferent signaling in the central nervous system. Quantitative RT-PCR revealed significant expression of histamine receptor subtypes (Hrh1– Hrh3) in mouse lumbosacral dorsal root ganglia (DRG), bladder detrusor smooth muscle, mucosa, and isolated urothelial cells. In DRG, Hrh1 was the most abundantly expressed. Acute histamine exposure evoked Ca2+ influx in select populations of DRG neurons but did not elicit calcium transients in isolated primary urothelial cells. Histamine-induced mechanical hypersensitivity ex vivo was abolished in the presence of the histamine H1 receptor antagonist pyrilamine and was not present in preparations from mice lacking transient receptor potential vanilloid 1 (TRPV1). Together, these results indicate that histamine enhances the sensitivity of bladder afferents to distension via interactions with histamine H1 receptor and TRPV1. This hypersensitivity translates to increased sensory input and activation in the spinal cord, which may underlie the symptoms of bladder hypersensitivity and pain experienced in IC/BPS. [ABSTRACT FROM AUTHOR]

Key points: Voltage‐gated sodium channels are critical for peripheral sensory neuron transduction and have been implicated in a number of painful and painless disorders.The β‐scorpion toxin, Cn2, is selective for NaV1.6 in dorsal root ganglion neurons.NaV1.6 plays an essential role in peripheral sensory neurons, specifically at the distal terminals of mechanosensing fibres innervating the skin and colon.NaV1.6 activation also leads to enhanced response to mechanical stimulus in vivo.This works highlights the use of toxins in elucidating pain pathways moreover the importance of non‐peripherally restricted NaV isoforms in pain generation. Peripheral sensory neurons express multiple voltage‐gated sodium channels (NaV) critical for the initiation and propagation of action potentials and transmission of sensory input. Three pore‐forming sodium channel isoforms are primarily expressed in the peripheral nervous system (PNS): NaV1.7, NaV1.8 and NaV1.9. These sodium channels have been implicated in painful and painless channelopathies and there has been intense interest in them as potential therapeutic targets in human pain. Emerging evidence suggests NaV1.6 channels are an important isoform in pain sensing. This study aimed to assess, using pharmacological approaches, the function of NaV1.6 channels in peripheral sensory neurons. The potent and NaV1.6 selective β‐scorpion toxin Cn2 was used to assess the effect of NaV1.6 channel activation in the PNS. The multidisciplinary approach included Ca2+ imaging, whole‐cell patch‐clamp recordings, skin–nerve and gut–nerve preparations and in vivo behavioural assessment of pain. Cn2 facilitates NaV1.6 early channel opening, and increased persistent and resurgent currents in large‐diameter dorsal root ganglion (DRG) neurons. This promotes enhanced excitatory drive and tonic action potential firing in these neurons. In addition, NaV1.6 channel activation in the skin and gut leads to increased response to mechanical stimuli. Finally, intra‐plantar injection of Cn2 causes mechanical but not thermal allodynia. This study confirms selectivity of Cn2 on NaV1.6 channels in sensory neurons. Activation of NaV1.6 channels, in terminals of the skin and viscera, leads to profound changes in neuronal responses to mechanical stimuli. In conclusion, sensory neurons expressing NaV1.6 are important for the transduction of mechanical information in sensory afferents innervating the skin and viscera. Key points: Voltage‐gated sodium channels are critical for peripheral sensory neuron transduction and have been implicated in a number of painful and painless disorders.The β‐scorpion toxin, Cn2, is selective for NaV1.6 in dorsal root ganglion neurons.NaV1.6 plays an essential role in peripheral sensory neurons, specifically at the distal terminals of mechanosensing fibres innervating the skin and colon.NaV1.6 activation also leads to enhanced response to mechanical stimulus in vivo.This works highlights the use of toxins in elucidating pain pathways moreover the importance of non‐peripherally restricted NaV isoforms in pain generation. [ABSTRACT FROM AUTHOR]